A generic optoelectronic device is known from H. P. Roser, R. U. Titz, G. W. Schwaab and M. F. Kimmit, Journal of Applied Physics, vol. 72, pages 3194 to 3297 (1992) which has the form of a point diode and is used as a heterodyne radiation detector. For this purpose, this diode comprises an antenna for coupling in a signal to be measured, wherein the antenna is in connection with the doped semiconductor layer via the formation of a Schottky point contact with a cross-sectional area A. This Schottky diode is used as a mixer and has the function of transforming a signal of frequency .nu..sub.sig to be measured to a substantially lower intermediate frequency or difference frequency .nu..sub.IF =.vertline..nu..sub.sig -.nu..sub.LO .vertline. by means of a reference radiation source of frequency .nu..sub.LO which serves as a local oscillator (LO). The resulting signal can then be amplified by means of conventional amplifiers and, if desired, spectrally analysed by conventional spectrometric systems.
A coherent microwave source or an optically pumped gas laser have been used as the reference radiation source.
Spectrometric systems comprising a radiation detector as described in the cited publication have been successfully employed as telescopes for approximately 10 years, both in ground stations and in aeroplanes.
It is known from the cited publication (JAP, 72, 3194 (1992)) that for a heterodyne detector for which the current constriction is realised by means of a Schottky point contact, the electrons flow through this conduction constriction ballistically and in packets when the Schottky diode which is operating as a detector is illuminated by the reference radiation source.
Ballistic transport occurs even at room temperature, since the electrons do not suffer from any inelastic scattering processes as they flow through the conduction constriction. Consequently, the electrons remain in phase with the incoming radiation of the reference radiation source as they pass through the conduction restriction.
Packet-wise transport of the electrons means that in each period of the reference source a particular number of electrons N.sub.e pass through the conduction constriction, i.e. through the Schottky contact region. The number N.sub.e of electrons per packet is independent of the frequency of the reference source. Therefore, with a measurement current I flowing through the diode the following equations holds EQU I=(N.sub.e e).nu..sub.LO ( 1)
e being the electron charge.
Up until now it was assumed that the diode in the region of the Schottky contact functioned in the manner of a capacitor which, in each period of the radiation originating from the reference source, was charged with a particular number N.sub.e of electrons and discharged once again. The depth D.sub.depl of the depletion zone which arises due to this charging and discharging process in the doped semiconductor layer bounding the Schottky contact is then given by EQU D.sub.depl =N.sub.e /(N.sub.d *A) (2)
with N.sub.d being the doping density of the doped semiconductor layer (epitaxial layer).
The ultimate sensitivity of detection achievable with a diode of this kind is of central importance and this ultimate detection sensitivity primarily depends on the noise produced in the diode itself.
It is experimentally established that maximum sensitivity (i.e. minimum noise) in the diodes studied is achieved at a particular current I.sup.Opt, the value of which being different from diode to diode. When a diode is driven at its "optimum" current I.sup.Opt then according to equation (1) a particular "optimum" number of electrons N.sub.e.sup.Opt per cycle flows through the Schottky contact. In the optimized case equation (1) thus becomes EQU I.sup.Opt =(N.sub.e.sup.Opt e).nu..sub.LO
In the prior art it was unclear why the maximum sensitivity of the detector occurred for a particular "optimum" number of transmitted electrons per period.
Correspondingly, the origin of the occurrence of an optimum measurement current flow was not understood and the values for this optimum measurement current flow could not be predicted in advance.
The fact that the depletion depths D.sub.depl calculated from equation (2) for various detectors which had different optimum current strengths were always substantially the same could also not be explained.
Alongside the selection of the measurement current, the intrinsic noise of the detector is also influenced by the selection of the diode parameters D (thickness of the doped semiconductor layer), N.sub.d (doping density of the doped semiconductor layer) and A (cross-sectional area of the doped semiconductor layer at the Schottky contact).
The detector can be described to a first approximation as a capacitance C.sub.j (which is a function of the voltage across the diode) and a non-linear diode resistance R.sub.j connected in parallel thereto together with a serial resistance R.sub.s connected in series to the diode. Accordingly, the diode capacitance C.sub.j is given to an approximation by the equation EQU C.sub.j =.epsilon..epsilon..sub.0 A/D.sub.depl ( 3)
wherein .epsilon. is the dielectric constant of the epitaxial layer and .epsilon..sub.0 the dielectric field constant, i.e. the permittivity of a vacuum.
The critical frequency .nu..sub.c0 (cut-off frequency) of the detector is then given by EQU .nu..sub.CO =(2.pi.R.sub.s C.sub.J).sup.-1 ( 4)
Increasing the cut-off frequency increases the separation of the frequency of the reference source and of the signal and thus improves the general noise properties of the detector.
In the prior art, the equation C.sub.J =.epsilon..epsilon..sub.0 A/D.sub.depl was understood to teach that in order to provide a low noise detector, the area A of the Schottky contact should be as small as possible and that the depths D.sub.depl of the depletion region should be as large as possible.
For this reason it was attempted to improve detector properties by reducing the contact area A, by lowering the doping density N.sub.d and by increasing the thickness of the epitaxial layer D. However, extensive and time consuming experimental investigations showed that improvements in the detectors of the prior art were no longer achievable in a controlled manner. In particular, it was unclear how the parameters of the detector A, D, N.sub.d which can be selected prior to production of the diode should be selected such that when, after production of the diode, the experimental optimization via establishment of the optimum current could result in a detector with an overall improved internal noise. It was also clear that no further optimization of the detectors was possible using equation (3).
Furthermore, it was established that by cooling generic diodes from for example 300K to 20K no significant improvement in the noise properties resulted. This stood in contrast to otherwise usual experience in the microwave, far infrared and infrared regions where such a cooling produces an improvement of at least a factor 2 to 4. Due to the very high current densities of approximately one million amperes per square centimeter (10.sup.6 A/cm.sup.2) through the Schottky contact area it would indeed have been expected that cooling the detector would have produced a substantially larger improvement in the noise.
From the above comments it is apparent that in the prior art it was unclear what was holding up the further development of the generic detectors being studied.